Tuesday, 22 October 2013

Ketchup and cornflour

Thixotropy, shear-thinning, refers to the process when a gel or sludge liquifies when shaken or stirred.  You know it best in tomato ketchup where a stout thump on the base of the bottle will slide some of the contents onto your chips. Many printers inks are thixotropic so that they can be squirted out of a nozzle as a liquid but not roll about the page after they arrive. You rely on it more essentially as a property of the synovial fluid in your joints - when the joint flexes it lubricates, when movement ceases it supports.  In the natural world, this is similar to quicksand which looks solid enough when at rest but liquifies when sheared by your foot. Thixotropy in the clay layer was probably an element in the acceleration of the mountain-side into the lake at Vajont.  Christchurch, NZ mostly situated on a flat coastal plain astride the Avon River sustained significant damage from liquefaction of subsoil under buildings during the 2011 earthquake.  For the same reason Boston, large tracts of which are built on landfill in Back Bay and similar areas, has one of the highest ratings in the USA for earthquake disaster potential.  Earthquakes are very rare on the seismically stable East coast, but if one should occur it Boston will be like a house of cards in a gale.

The opposite of thixotropic is a dilatant fluid that becomes thicker when sheared, shoved, scooped or stirred.  The best example of this (lots of fun in the kitchen) is a mix of cornstarch and water that you can scoop quickly up in your hand and then have run through your fingers - preferably back into the bowl. Or you can scale up - but you have to be quick!  If you didn't, like me, fail Ordinary Level Physics check out "non-Newtonian fluids - you'll be able to understand it.

On 21st October 1966, the primary school in the village of Aberfan situated in a valley in the heart of the South Wales coalfield near Merthyr Tydfil was engulfed in wave of sludge and rock that came down the mountainside through the fog with a roar sounding like a jet-plane crashing.  116 children and 28 adults were crushed to death or suffocated under tons of loose spoil from the local coal-mines which had been casually dumped on the uplands by the National Coal Board.  Out of ignorance and negligence at least some of these spoil heaps were dumped on natural springs and in any case there is a lot of rain in the Welsh valleys.  What seems to have happened is that one face of a slag-heap slumped a few meters after days of heavy rain and that shearing shock caused the rest of the saturated artificial mountain to liquefy like ketchup.  The total volume shifted was about 150,000 cu.m but 40,000 cu.m of that finished in and on the village school - it was buried 10m deep in places.

In the aftermath, Lord Robens, chairman of the National Coal Board, behaved in a way that was not so much shabby as callous and duplicitous, as well as rapacious in the very worse tradition of 19th century robber-barons.  It brought home to thoughtful people that, for all the high-minded aspirations of the left, there really was little difference between capitalism and state socialism in their actions on the ground and in tribunals. Robens started off as a leftist MP for a mining constituency in Northumberland and a minister in the 1945 Labour government.  He used his appointment to the NCB by the Conservative government of Harold Macmillan to transform himself into director of this and chairman of that as if he'd been born to it.  I don't think it's insane to suggest that Robens sowed the seeds of disillusion that sprouted in the election of Margaret Thatcher's Conservative government 12 and a half years later.

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